Gas detector and method of operating a gas detector

ABSTRACT

The invention relates to an infrared gas sensor with an energy supply apparatus for operating at least one radiation source with current or voltage pulses, with at least one measurement area disposed in the beam path, with at least one wavelength-selecting element, with at least one detector element emitting an electrical measurement signal and with a switching device ( 3 ) to control the pulse duration of the current or voltage pulses. 
     The switching device ( 3 ) has means for setting the pulse duration in such a way that the current or voltage pulse is so turned off that the required pulse duration is smaller than that required to reach the maximum (τ max ) of the at least one measurement signal of the at least one detector element.

BACKGROUND OF THE INVENTION

Infrared gas sensors are based on the principle of the selectiveabsorption of infrared radiation by gases. The action principle and theoperating manner of infrared gas sensors may be assumed as known.

Such gas sensors generally comprise one or more radiation sources, e.g.thermal radiators such as incandescent lamps, one or more absorptionsections, wavelength-selecting elements (selective radiation filterssuch as e.g. interference filters) and one or more radiation detectors,which convert the optical signal into an electrical measurement signal.A large number of such detectors is known from prior art. The mostfrequently used types of detector are inter alia pyroelectric,semiconductor (e.g. on a PbSe base) thermophile and pneumatic detectors.

A further type of embodiment is based on the alteration in pressure as aresult of the heating of the gas molecules in the measurement area,which is detected by means of a microphone. The heating is caused by theabsorption of the radiation energy of the radiation source(s) by themeasurement gas molecules to be detected. An embodiment corresponding toprior art is disclosed in the patent DE 195 25 703 A1.

Thus, U.S. Pat. No. 5,608,219 teaches a device for detecting at leastone gas with an absorption band in the infrared range, comprising a cellwhich contains the gas sample to be tested, an infrared radiationsource, a power supply for the radiation source, an infrared radiationsensor and a measured-value evaluation unit which is secured to thesensor.

EP 512 535 teaches a portable carbon dioxide monitor in which the carbondioxide is determined by non-dispersive infrared measurements.

EP 503 511 teach a device for the analytical determination of carbondioxide and water through infrared analysis techniques. The gas analyzerhere contains a radiation source, a sample cell and a reference cell, adetector and a gas supply.

U.S. Pat. No. 5,734,165 describes an infrared photometer which isintegrated in a compact housing in which an infrared radiation source isprovided with a pulse frequency of between 0.01 and 10 Hz.

U.S. Pat. No. 4,914,720 teaches a gas analyzer which is based onnon-dispersive infrared photometry and with which different gases in gasmixtures can be determined.

In optical gas sensors, the radiation source is generally operatedcontinuously (in combination with a chopper) or intermittently with apulsed voltage. In both cases, the signal is generally detected by meansof a phase-sensitive electronics (lock-in technology) or respectively anRMS converter.

The invention relates to an infrared gas sensor with an energy supplyapparatus for operating at least one radiation source, for example aheat radiation source or an infrared luminescence radiation source (e.g.diode, laser diode, infrared laser), with current or voltage pulses,with at least one measurement area arranged in the beam path with atleast one detector emitting an electrical measurement signal and withone switching device to control the pulse duration of the current orvoltage pulses. The invention relates furthermore to a method ofoperating such a sensor.

In a gas sensor of this type, known from DE 30 43 332 A1, the pulseduration of the radiation source, a heat radiator, is controlled independence on the maximum value of the measurement signal of a radiationdetector at the exit of the measuring transducer. It is proposed herethat the maximum value be obtained empirically or mathematically fromthe course of the response curve or of the measurement signal of themeasuring transducer, and that the pulse duration be controlled, byinterrupting the energy supply to the heat radiator, in such a way thatthe maximum value lies with certainty within the pulse duration.Furthermore it is proposed that the course of the response curve bemonitored by means of a maximum value detector which interrupts theenergy supply or respectively the current pulse when the maximum valueof the response curve is reached. With this gas sensor, a pulse/pauseratio of the energy supply of <1 and preferably between 0.1 and 0.5 canbe achieved.

SUMMARY OF THE INVENTION

The object underlying the invention is to create a gas sensor of thistype which has a reduced energy consumption with high measuringaccuracy.

This object is achieved in relation to the sensor by the features ofpatent claim 1 and in relation to the method by the features of claim 9.The subordinate claims list advantageous developments.

According to the invention, therefore, the switching device is socontrolled that the pulse duration is smaller than the time intervaluntil the measurement signal has reached the maximum at τ_(MAX). Thiscan come about by the maximum rise of the detector signal$\left( \frac{{U_{0}(t)}}{t} \right)_{MAX}$

being used as the measured value. The maximum rise of the signal isachieved substantially quicker than its maximum value: τ′<<τ_(MAX).Therefore the turn-on time of the radiation source τ₀ in thismeasurement process can be substantially shorter τ′≦τ₀<<τ_(MAX) than inall the previous ones. The pulse/pause ratio which can be thus realisedis below 0.01.

The interval between individual measurements can be selected accordingto sensor design and application profile between a few seconds andseveral minutes, whereby it is possible for an operating period withouta change of batteries (e.g. standard AA batteries) of more than half ayear to be achieved.

This method according to the invention, like the methods according tothe invention presented below, can be carried out both with a radiationdetector and with an acoustic detector, such as e.g. a microphone, asthe detector element. Simultaneously with a plurality of detectors, e.g.a radiation detector and a microphone, a plurality of signals can beproduced, the measurement being carried out according to the inventionfor at least one of the two signals or also for a plurality of thesignals or respectively for all the signals. When a plurality of signalsis produced, the accuracy of the measurement can be further improved.

Suitable as radiation sources for infrared rays are for exampleincandescent lamps, thermal thin-film radiators, light emitting diodesand the like which can also be used as pulsed infrared radiationsources.

Another method of shortening the turn-on time of the radiation source,is using the measurement of the detector signal at a specific time τafter turning on the radiation source as the sensor signal, this time τbeing smaller than the time of reaching the maximum value of thedetector signal τ_(MAX): τ<τ_(MAX). Preferably τ lies between the timeof reaching the maximum value of the rise of the detector signal τ′ andτ_(MAX): τ′≦τ<τ_(MAX). As the measured value at the time τ, both theinstantaneous value of the detector signal U_(D)(τ) and theinstantaneous value of its first derivative can be used$\left( \frac{{U_{D}(t)}}{t} \right)_{\tau = \tau^{\prime}}$

Furthermore it is also possible to use as the measured value the periodof time after the turn-on of the radiator in which a specific level ofthe detector signal U″ or a specific spacing from the detector signalwhen the radiator is turned off (offset) is reached.

The period of time until a fixed rise of the detector signal is reachedcan also be used as the measurement signal.

Very advantageous is the operating manner of the gas sensor when themaximum rise of the detector signal$\left( \frac{{U_{D}(t)}}{t} \right)_{MAX}$

or the time at which this maximum value is reached τ′ is used as themeasurement signal. Here the maximum rise can be measured with the aidof a maximum value detector without giving the time of measurement.

The time τ′ can be determined e.g. with the aid of a zero crossingdetector from the second derivative of the detector signal.

Measuring the integral of the detector signal up to a specific point oftime τ, e.g. up to the maximum of the rise of the detector signal τ′, isa further possible measuring method.

Preferably that is an integral over a period of time starting from theturn-on point of the radiation source (t=0) up to a specific time τwhich is smaller than the pulse duration τ<τ₀: ∫₀^(τ)U_(D)(t)t.

Naturally also a plurality of the above-described methods and measuredvalues (e.g. $\left( \frac{{U_{D}(t)}}{t} \right)_{MAX}$

and τ′) can be combined with one another to increase the sensitivity orthe reliability.

The invention also relates to a method of operating an infrared gassensor such as has been explained in detail above.

The method according to the invention is carried out in such a way thatthe current or voltage pulse is so turned off that the pulse duration issmaller than that required to reach the maximum (τ_(MAX)) of themeasurement signal of the radiation detector(s) or of the acousticdetector. Preferred embodiments of the method consist in the current orvoltage pulse being turned off after a time t=τ, τ lying between t=0 ofthe current or voltage pulse and τ_(MAX). It is particularly preferredif the current or voltage pulse is turned off at the maximum value ofthe first derivative of the measurement signal within the pulseduration. But all the embodiments previously explained in connectionwith the sensor can be used in the method according to the invention.

The method according to the invention has additional advantages. It isthus also possible with the method according to the invention todetermine the gas concentration in a different manner. Thus the gasconcentration can be determined from the value of the first derivativeof the measurement signal at a specific time τ_(meβ), which is smallerthan the pulse duration or at a value of the N-th derivative (N>1) ofthe measurement signal at a specific time τ_(meβ), which is smaller thanthe pulse duration. The gas concentration can furthermore be determinedfrom the value of the integral of the measurement signal over a periodof time starting from the turn-on time t=0 up to a specific time τ_(meβ)which is smaller than the pulse duration. The first derivative can alsobe used for determining the gas concentration. Further preferredembodiments of the method for determining the gas concentration arequoted in claims 16 to 19.

The method according to the invention can also be used when the timeconstant of the detector is greater than the time constant of theradiation source used. It includes furthermore measuring methods andmeasuring arrangements suitable for same, in which the measurementsignal dependent on the gas concentration is determined before themaximum of the detector signal is reached. In particular the pulselength of the voltage applied to the radiation source to operate samecan be kept so short that the maximum of the detector signal is notreached. For frequently the kinetic course of the detector outputsignal, e.g. in the case of pyroelectric detectors, is determined morestrongly by the properties, such as for example the time constant, ofthe detector itself than by the properties of the radiation source.

BRIEF DESCRIPTION OF THE DRAWINGS

In what follows a few examples of the method according to the inventionof the gas sensor according to the invention are described. The figuresshow:

FIG. 1 a typical signal shape for a pyroelectric radiation detector;

FIG. 2 a basic signal shape of a pyroelectric detector where there is apulsed radiation source;

FIG. 3 a derivative circuit;

FIG. 4 a summing circuit;

FIG. 5 the representation of a summing measurement;

FIG. 6 a calibration curve of an infrared CO₂ sensor;

FIG. 7 the dependency of the maximum value of the derivative of a sensorsignal of an infrared CO₂ sensor on the turn-on time of the radiationsource and

FIG. 8 the course of the derivatives of the sensor signal of apyroelectric detector for very short turn-on times.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 is represented a typical signal shape for a radiationdetector, here a pyroelectric detector.

The radiation source was here turned on at the time t=0. Curve 1reproduces the course of the output signal of the detector (minusoffset). Curves 2 and 3 show the course of the first or the secondderivative.

FIG. 2 shows the basic signal shape of a pyroelectric detector (MurataIRA410 QW1), which is exposed to the radiation of a pulsed radiationsource (VCH T1 5 V, 60 mA). In FIG. 2 a group of curves is shown, thetotal turn-on time of the radiation source being used as a parameter. InFIG. 2 it can be recognised that the output signal of the pyroelectricdetector requires approximately 2.5 seconds as a reaction to theswitching on of a radiation source until it reaches its maximum. Themaximum value of the first derivative of the detector signal is howeverreached after only 120 milliseconds (compare below, FIG. 8). Iftherefore the maximum value of the first derivative is used to determinethe sensor signal, the radiation source can be turned off much earlier,such that the energy consumption of the radiation source is alsoconsiderably reduced. Moreover the detector signal drops after such ashort radiation pulse very much quicker to its normal level, which meansthat higher scanning rates without significant heating of the sensor andthus without heating-up time for the sensor can be used.

FIG. 3 shows a derivative circuit, by means of which the maximum valueof the derivative of the sensor signal can be detected. In thisderivative circuit, a microprocessor 2 controls a pulse generator 3 insuch a way that the latter turns on the radiation source of a sensor 1for a defined time. This time can be either rigidly set or can depend onthe feedback from a peak value detector 6 to the microprocessor 2. Thesensor 1 produces a measurement output signal by means of a radiationdetector. This output signal passes through a low pass 4 in order tosuppress higher frequency noise and a differentiator 5 in order todifferentiate the measurement output signal electronically. This signalis input into a peak value detector 6, which stores the peak value ofthe derivatives and possibly, as described above, sends to themicroprocessor 2 feedback on the attainment of the peak value. Thederivative peak value detected by the peak value detector is passed toan analogue-to-digital converter 7 which converts and measures thepresent maximum value of the derivative of the sensor signal.Furthermore the signal of a temperature sensor contained in the sensor 1is detected by the analogue-to-digital converter 7. Both values are thenpassed by the analogue-to-digital converter 7 to the microprocessor 2.The latter then resets the peak value detector and calculates from themeasured values, with the aid of corresponding calibration functions,the concentration of the gas to be measured. The result is thenindicated by the microprocessor 2 at an output or on a display 8.

FIG. 4 shows a summing circuit, in which a microprocessor 2 as in FIG. 3drives a pulse generator 3 which turns on a radiation source of a sensor1 for a defined time. The output signal of the radiation detector of thesensor 1 is passed to a subtractor 10, by any possible offset of thesensor signal being subtracted from the sensor signal. The voltage to besubtracted is provided to the subtractor 10 by the microprocessor 2 viaa digital-to-analogue converter 13. The sensor signal thus freed of theoffset is passed to a low pass 11 in which it is low-pass filtered andthen received by an analogue-to-digital converter 12, converted intodigital signals and passed to the microprocessor 2 which sums above acertain number of values.

If the offset of the sensor signal is temperature-dependent, it isfurthermore possible to detect with the analogue-to-digital converter 12any offset still present after the subtractor 10 before the radiationsource is turned on. This comes about by the analogue-to-digitalconverter 12, before the radiation source is turned on, receiving thelow-pass filtered offset of the sensor signal, converting it intodigital signals and passing it also to the microprocessor 2, which sumsabove a specific number of values. The analogue-to-digital converter 12can moreover measure the output of the temperature sensor contained insensor 1 and pass this output to the microprocessor 2. Themicroprocessor 2 then calculates from the measured values, with the aidof a corresponding calibration function, the concentration of the gas tobe measured.

FIG. 5 shows the course of a summing measurement, in which the offset ofthe sensor signal (detector: Murata IRA410 QW1) is determined by asummation of 2048 measurements of an analogue-to-digital converter (MAX1246). The analogue-to-digital converter here measures at a rate of16,000 measurements per second. These measurements come about before thelight source (VCH T1 5 V, 60 mA) is turned on. After the light source isturned on, there is a pause of about 125 milliseconds and then again2048 measurements are carried out by the analogue-to-digital converterand are added up by a microprocessor. These values then form the actualinfrared measured values. To calculate the concentration of the gas tobe determined, the difference from these measured values and themeasured offset is evaluated with the aid of a calibrating function.

FIG. 6 shows the calibration curve of an infrared CO₂ sensor (detector:Heimann LHi 807-TC-G2; source: VCH T1 5 V 125 mA; light path 25 mm) inthe range between 0 ppm and 2100 ppm CO₂. The linear calibrationfunction is here optimised for the range between 800 ppm and 1200 ppmCO₂. This calibration curve was recorded with a derivative circuitaccording to FIG. 3. It can be recognised that the signal intensity hasbeen correlated with the CO₂ concentration and consequently a veryreliable CO₂ sensor has been realised.

FIG. 7 shows the dependency of the maximum value of the derivatives ofthe detector signal of a pyroelectric infrared detector (Heimann LHi807-TC-G2) on the turn-on period of the radiation source. A miniatureincandescent lamp (VCH T1 5 V, 60 mA) was used here as the radiationsource. As can be recognised, the maximum value of the derivative risesin the range between 40 milliseconds and 150 milliseconds and no longeralters the pulse duration above 150 milliseconds. Turning on theradiation source for more than 160 milliseconds thus only increases theenergy consumption of the sensor, but does not contribute anything moreto the concentration-dependent derivative output signal according to theinvention.

FIG. 8 shows the course of the first derivative of the detector signalof a pyroelectric detector (Murata IRA410 QW1) with very short turn-ontimes. FIG. 8 here shows a group of curves, the turn-on time of thelight source (VCH T1 5 V 60 mA) being used as the group parameter. Thisamounts to between 20 milliseconds and 630 milliseconds. It can berecognised that the maximum value of the first derivative of thedetector signal for a pulse duration of the light source rises from 20milliseconds to 160 milliseconds. Longer pulse widths than 160milliseconds do not lead to any further increase in the maximum of thefirst derivative of the detector signal.

What is claimed is:
 1. An infrared gas sensor with an energy supplyapparatus for operating at least one radiation source with current orvoltage pulses, with at least one detector element emitting anelectrical measurement signal, with at least one wavelength-selectingelement located between the at least one radiation source and the atleast one detector element, and with at least one measurement areaarranged between the at least one radiation source and the at least onedetector element and disposed in the beam path, and with a switchingdevice to control the pulse duration of the current or voltage pulses,wherein the switching device has means for setting the pulse duration insuch a way that the current or voltage pulse is turned off in a mannersuch that the pulse duration is smaller than the required to reach themaximum (τ_(MAX)) of the at least one measurement signal of the at leastone detector element and such that the turn-off takes place when aspecific measurement signal value is reached within the pulse durationor the turn-off occurs when a specific value of the first derivative ofthe measurement signal is reached within the pulse duration or theturn-off occurs when the maximum value of the first derivative of themeasurement signal is reached within the pulse duration or the turn-offoccurs when a specific value of the integral of the measurement signalover a time interval, starting from the tun-on time t=0 is reachedwithin the pulse duration.
 2. An infrared gas sensor according to claim1, wherein the at least one detector element is at least one of aradiation detector, a pressure transducer and a microphone.
 3. Aninfrared gas sensor according to claim 1, wherein the current or voltagepulse is turned off after a specific time t=τ, the time τ lying betweenthe turn-on time t=0 of the current or voltage pulse and τ_(MAX).
 4. Aninfrared gas sensor according to claim 1, wherein the switching deviceis so operated that a pulse/pause ratio of less than 0.01 is set.
 5. Amethod for operating an infrared gas sensor according to claim 1,wherein the current or voltage pulse is turned off in a manner such thatthe pulse duration is smaller than that used to reach the maximum(τ_(MAX)) of the measurement signal of the at least one detector elementand such that the turn-off takes place when a specific measurementsignal value is reached within the pulse duration or the turn-off occurswhen a specific value of the first derivative of the measurement signalis reached within the pulse duration or the turn-off occurs when themaximum value of the first derivative of the measurement signal isreached within the pulse duration or the turn-off occurs when a specificvalue of the integral of the measurement signal over a time interval,starting from the turn-on time t=0 is reached within the pulse duration.6. A method according to claim 5, wherein, to determine the gasconcentration, the value of the first derivative of the measurementsignal at a specific time τ_(meβ) which is smaller than the pulseduration, is used.
 7. A method according to claim 5, wherein, todetermine the gas concentration, the value of an N-th derivative (N>1)of the measurement signal at a specific time τ_(meβ) which is smallerthan the pulse duration, is used.
 8. A method according to claim 5,wherein to determine the gas concentration the value of the integral ofthe measurement signal over a period of time starting from the turn-onpoint t=0 up to a specific time τ_(meβ) which is smaller than the pulseduration, is used.
 9. A method according to claim 5, wherein todetermine the gas concentration, the maximum value of the firstderivative of the measurement signal is used.
 10. A method according toclaim 5, wherein, to determine the gas concentration, at least one ofthe times, is used at which a specific value of the measurement signal,of the N-th derivative (N>1) of the measurement signal or of theintegral of the measurement signal is reached.
 11. A method according toclaim 5, wherein, to determine the gas concentration, the time at whichthe maximum value of the first derivative of the measurement signal isreached, is used.
 12. A method according to claim 5, wherein, todetermine the gas concentration, the time of the first zero crossing ofthe second derivative of the measurement signal is used.
 13. A methodfor operating an infrared gas sensor according to claim 5, to determinethe concentration of a gas, comprising evaluating the results of atleast two tests of the same measurement signal, said at least two testsbeing selected from the group consisting of: a) using the value of themeasurement signal at a specific time τ_(meβ), which is smaller than thepulse duration; b) using the value of the first derivative of themeasurement signal at a specific time τ_(meβ), which is smaller than thepulse duration; c) using the value of an N-th derivative (N>1) of themeasurement signal at a specific time τ_(meβ), which is smaller than thepulse duration; d) using the value of the integral of the measurementsignal over a period of time starting from the turn-on point t=0 up to aspecific time τ_(meβ), which is smaller than the pulse duration; e)using the maximum value of the first derivative of the measurementsignal; f) using at least one of the times, at which a specific value ofthe measurement signal, of the N-th derivative (N>1) of the measurementsignal or of the integral of the measurement signal; g) using the timeat which the maximum value of the first derivative of the measurementsignal is reached; and h) using the time of the first zero crossing ofthe second derivative of the measurement signal.